NAME¶

std::type_identity - std::type_identity

Synopsis¶

Defined in header <type_traits>
template< class T > (since C++20)
struct type_identity;

Provides the member typedef type that names T (i.e., the identity transformation).

The behavior of a program that adds specializations for type_identity is undefined.

Member types¶

Name Definition
type T

Helper types¶

template< class T > (since C++20)
using type_identity_t = typename type_identity<T>::type;

Possible implementation¶

template< class T >
struct type_identity {
using type = T;
};

Notes¶

type_identity can be used to establish non-deduced contexts in template argument
deduction.

Feature-test macro: __cpp_lib_type_identity

Example¶

// Run this code

#include <iostream>
#include <type_traits>

template <class T>
T foo(T a, T b) {
return a + b;
}

template <class T>
T bar(T a, std::type_identity_t<T> b) {
return a + b;
}

int main() {
// foo(4.2, 1); // error, deduced conflicting types for 'T'
std::cout << bar(4.2, 1) << '\n'; // OK, calls bar<double>
}

Output:¶

5.2